1. Field of Invention
The invention relates to a catadioptric projection lens for imaging a pattern arranged in an object plane onto an image plane.
2. Description of the Related Art
Projection lenses of said type are employed on projection illumination systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of micro-devices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh-resolution on a reduced scale.
In order to create even finer structures, it will be necessary to both increase the numerical aperture (NA) of the projection lens to be involved on its image side and to employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm.
However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, such as calcium fluoride, magnesium fluoride, barium fluoride, lithium fluoride, lithium calcium aluminum fluoride, lithium strontium aluminum fluoride, and similar, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbé numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that have been sufficiently well-corrected for chromatic aberrations. Although this problem could be solved by employing purely reflective systems, fabricating such mirror systems requires substantial expense and effort.
In view of the aforementioned problems, catadioptric systems that combine refracting and reflecting elements, i.e., in particular, lenses and mirrors, are primarily employed for configuring high-resolution projection lenses of the aforementioned type.
Whenever imaging reflective surfaces are employed, it will be necessary to use beam-deflecting devices if images free of obscurations and vignetting are to be achieved. Both systems having one or more deflecting mirrors and systems having solid beam-splitters are known. Additional plane mirrors may also be employed for folding the optical path. Folding mirrors are usually employed only in order to allow meeting space requirements, in particular, in order to orient the object and image planes parallel to one another. However, folding mirrors are not absolutely necessary from the optical-design standpoint.
Employing systems having a solid beamsplitter in the form of, e.g., a beamsplitter cube (BSC), has the advantage that it allows implementing on-axis systems. Polarization-selective reflective surfaces that either reflect or transmit incident radiation, depending upon its predominant polarization direction, are employed in such cases. The disadvantage of employing such systems is that hardly any suitable transparent materials are available in the desired, large volumes. Moreover, fabricating optically active beamsplitter coatings situated within beamsplitter cubes is extremely difficult. Heating effects occurring within beamsplitters may also present problems at high radiant intensities, since inside the beamsplitters an intermediate image is created.
One example of such a system is depicted in European Pat. No. EP-A-0 475 020, which corresponds to U.S. Pat. No. 5,052,763, where the mask involved lies directly on a beamsplitter cube and the intermediate image formed lies within the beam-splitter cube, behind its internal beamsplitting surface. Another example is depicted in U.S. Pat. No. 5,808,805 and the associated application for continuation of same, U.S. Pat. No. 5,999,333, where a multi-element compound-lens group with a positive refractive power lies between the object plane and a beamsplitter cube. The collected light beam is initially deflected toward a concave mirror by the beamsplitter cube and then reflected back to the beamsplitter cube and through its beamsplitting surface toward the aforementioned compound-lens group with a positive refractive power by the concave mirror. The intermediate image lies within the beamsplitter cube, in the immediate vicinity of its beamsplitting surface. However, none of these documents makes any statements regarding heating problems that might arise or how they may be avoided.
European Patent No. EP-A-0 887 708 states measures for avoiding thermally induced imaging errors for a catadioptric system having a beamsplitter cube, but apparently no intermediate image falling within its beamsplitter cube. The intention here was obtaining a symmetric distribution of radiant intensity over the beam-splitter cube's beamsplitting surface, i.e., a distribution that would yield a heating profile symmetrically distributed over the beam-splitter's beamsplitting surface, by suitably routing the beam transiting the beamsplitter cube. It was stated that the resultant wave-front distortions, such as those that result from nonuniform heating, which are difficult to eliminate, were avoidable.
Some of these disadvantages of systems having beamsplitter cubes may be avoided in the case of systems having one or more deflecting mirrors in their beam-deflecting device. However, such systems have the disadvantage that they are, by virtue of their design, necessarily off-axis systems.
A catadioptric reduction lens of that type is described in European Pat. No.
EP-A-0 989 434, which corresponds to U.S. Ser. No. 09/364,382. These types of lenses have a catadioptric first section having a concave mirror and a beam-deflection device that is followed by a dioptric second section arranged between their object plane and their image plane. Their beam-deflecting device, which is configured in the form of a reflecting prism, has a first reflective surface for deflecting radiation coming from their object plane to a concave mirror and a second reflective surface for deflecting radiation reflected by that concave mirror to a second section containing exclusively refractive elements. Their catadioptric first section creates a real intermediate image that lies slightly behind this prism's second reflective surface and well ahead of the first lens of their second section. Their intermediate image is thus readily accessible, which may be taken advantage of for, e.g., installing a field stop.
Another reduction lens that has a beam-deflection device having a deflecting mirror is described in U.S. Pat. No. 5,969,882, which corresponds to European Pat. No. EP-A-0 869 383. This system's deflecting mirror is arranged such that light coming from its object plane initially strikes the concave mirror of its first section, where it is reflected to the system's beam-deflecting device's deflecting mirror, where it is reflected to a second reflective surface, where it is deflected toward the lens of the system's exclusively dioptric second section. The elements of this system's first section that are utilized for creating its intermediate image are configured such that its intermediate image lies close to its beam-deflecting device's deflecting mirror. Its second section refocuses its intermediate image onto its image plane, which may be oriented parallel to its object plane, thanks to the reflecting surface that follows its intermediate image in the optical train.
U.S. Pat. No. 6,157,498 depicts a similar configuration whose intermediate image lies on, or near, the reflective surface of its beam-deflecting device. Several lenses of its second section are arranged between its beam-deflecting device and a deflecting mirror located in its second section. In addition, an aspheric surface is arranged in the immediate vicinity of, or near to, its intermediate image exclusively for the purpose of correcting for distortions, without affecting other imaging errors.
A projection lens having a reducing catadioptric section and an intermediate image in the vicinity of the deflecting mirror of a beam-deflection device is depicted in German Pat. No. DE 197 26 058.
The U.S. patent mentioned above, U.S. Pat. No. 5,999,333, depicts another catadioptric reduction lens having deflecting mirrors for which light coming from its object plane initially strikes a concave mirror, where it is reflected to the lens' beam-deflecting device's sole reflective surface. The intermediate image created by its catadioptric section lies close to this reflective surface, which reflects light coming from that concave mirror to a dioptric second section that images this intermediate image onto its image plane. Both its catadioptric section and its dioptric section create reduced images.
A similarly configured lens for which the intermediate image created by its catadioptric section lies near its deflecting device's sole reflective surface is depicted in Japanese Pat. No. JP-A-10010429. The surface of the lens of the following dioptric section that lies closest to the deflecting mirror is aspheric in order that it may make a particularly effective contribution to correcting for distortions.
Those systems whose intermediate image lies near, or on, a reflective surface may be compactly designed. They also allow keeping the field curvatures of these systems, which are off-axis illuminated, that will need to be corrected small. One of their disadvantages is that even the slightest flaws on any of their reflective surfaces may adversely affect the qualities of images projected onto their image plane. Moreover, their focusing of radiant energy onto reflective surfaces may cause heating effects that might adversely affect their imaging performance. The resultant, locally high, radiant intensities may also damage the reflective coatings that are normally applied to the surfaces of mirror blanks.